Gate valves, and particularly motor-operated and air-operated gate valves are used extensively within nuclear power plants and process industries for the purpose of controlling fluid flow. The function of a gate valve is to provide positive flow isolation in fluid systems. Gate valves with motor- and air-operators (as well as other types of power operators) are used in critical safety-related functions in which the valves are automatically closed (or opened in some cases) in response to emergency conditions in order to ensure that the plant can be safely shutdown and accidents effectively mitigated.
In recent years, a variety of problems with motor-operated gate valves in particular has led to substantial expenditures by utilities, as well as the Nuclear Regulatory Commission (NRC), in efforts to ensure that these gate valves can be seated (or opened) under accident conditions. Two of the particular parameters of concern in making the determination of the valve's ability to function as required are internal clearances and delivered thrust. Various valve dimensions and clearances, such as the gate valve guide to disk clearance, T-bar to disk clearance, and the seat vs. guide limited angles are critical to successful valve performance. The thrust delivered to the disk by the operator through the stem is also critical to successful valve performance.
A wide variety of valve and valve operator problems have been documented. Probably the most visible issues have been addressed in the various revisions of NRC Bulletin 85-03, and in NRC Generic Letter 89-10, which deal with motor-operated valve (MOV) problems, and requires utilities to take actions to ensure that their MOVs will function as required. While the level of attention given to MOVs has been high, some of the same considerations apply to valves powered by other types of operators.
It is thus in the interest of the utility, and to the general public, that gate valves be monitored for degradation due to service wear. A continuing need exists for a reliable method and system for carrying out such monitoring so as to provide information on clearances, wear and developed thrust for valves used in critical safety-related applications, without requiring disassembly or other intrusive monitoring.
U.S. Pat. No. 5,029,597 to Leon describes a controller for a motor operated valve (MOV) in which a strain gauge is placed on the yoke of the valve. The strain gauge measures valve stem load and a monitor determines the rate of change of the stem load. When the rate of change reaches a threshold value, the motor is shut off to avoid overloading.
U.S. Pat. No. 4,844,110 to Paley describes a MOV having an overload sensor which measures current through the valve motor and controls operation of the motor in response to the sensed motor current.
European Patent Application No. 0 287 299 A2 describes a MOV diagnostic system which includes a sensor for determining valve stem position as it translates up and down. Actual and predicted positions are compared, as are actual stem load and predicted stem load.
None of the above-noted references provides an adequate system for determining, on a non-intrusive basis, seating force and/or degradation of a MOV, as reflected in gate valve clearances.